This invention relates generally to acoustic transducers and more particularly to flextensional underwater acoustic transducers.
As it is known in the art, a flextensional transducer typically includes a high strength oval shaped shell which flexes to propagate acoustic waves in a surrounding seawater medium. An electromechanical driver, disposed within the shell, is fed by an alternating current and expands and retracts in an oscillatory manner upon electrical energization to transmit like motions to end portions of the shell disposed along the major axis of the shell. The dynamic force provided by the expansion of the electromechanical driver exerted on end portions of the shell is superimposed on a static compressive bias on the electromechanical driver and causes shell portions along the minor axis of the shell to flex inward. The subsequent retraction of the electromechanical driver causes the shell portions along the minor axis of the shell to flex outward. This flexing action is repeated in an oscillatory manner to propagate acoustic waves in the surrounding seawater medium. Often, mechanical end blocks are positioned between the end portions of the shell and ends of the electromechanical driver adjacent to such end portions to couple the force provided by the electromechanical driver to the shell. End caps are located at opposite ends of the shell and seal the transducer so that seawater does not enter the shell housing. Generally, a flextensional transducer further includes rigid support members to provide mechanical integrity to the transducer and a central support structure to provide mechanical support to the electromechanical driver and to the end caps. The electromechanical driver may be referred to as a transduction driver of which the input energy is electrical waves or electrical energy, and the output energy is acoustic waves or acoustic energy.
As it is further known in the art, one type of electromechanical driver includes a plurality of piezoelectric ceramic elements disposed in a stack arrangement or assembly. The stack arrangement of the electromechanical driver has a length which, generally, significantly exceeds its width or height and thus the driver is susceptable to lateral bending due to shocks experienced by the transducer, such as in the case of a transducer which is rigidly mounted to a surface ship near which an explosive causes substantial shock waves in the surrounding seawater. While a central support structure is conventionally used to minimize the susceptibility of the stack assembly to potentially damaging shocks experienced by the transducer, it is important that such a support structure not restrict the unrestrained motion of the stack assembly upon electrical energization since such restriction can inhibit the efficiency of the propagation of acoustic energy.
One type of support structure known in the art for providing mechanical support to the stack assembly and to the end caps is an I-beam structure. In using an I-beam central support structure, the stack assembly is essentially divided into two stack portions, with a portion located and adhered, or fastened, to each side of the I-beam central support structure. Thus, the I-beam support structure maintains a first end of each of the stack portions in a stationary position with respect thereto, in order to prevent the transmission of acoustic energy into the rigid support member, such transmission decreasing the efficiency of the transducer.
The occurrence of explosive shock waves can cause substantial lateral forces on the shell. Since the ends of the stack portions adjacent to end portions of the shell will move laterally with such shock wave forces while the ends of the stack portions fastened to the I-beam structure remain stationary, lateral bending of the stack portions may result. Further, relatively high tensile stresses may occur on a convexly bent side of a stack portion in spite of the high compressive bias on the stack portions. High tensile stresses in the ceramic stack may generate cracks in the ceramic material, such cracks potentially resulting in a high electric discharge, or corona, resulting from ionization of the gas trapped within the cracks.
It would thus be desirable to minimize the tendency of the ceramic stack assembly to laterally bend in response to shock waves. This would minimize potential tensile stresses and concomitant damage to the stacks associated with such lateral bending. It would also be desirable to minimize such lateral bending while not inhibiting the unrestrained motion of the ceramic stack and shell which otherwise would affect transducer efficiency.
As it is also known in the art, heat dissipation within the electromechanical driver is a critical performance factor since excessive temperatures may degrade the piezo-electric properties of the ceramic elements of the stack. This would result in reduced transducer efficiency and output capability. Typically, the ceramic assembly must be maintained at a temperature of less than approximately 77.degree. C. in order to provide maximum transducer efficiency and output capability.
Several factors should be considered when addressing the problem of heat dissipation in a transducer. Specifically, the cooling should be accomplished without inhibiting the unrestrained motion of the electromechanical driver and the shell in order to maintain acceptable transducer efficiency. Additionally, the transducer should operate, and therefore be cooled, in multiple physical orientations. Further, ease of manufacturability and servicability should be provided.
As it is known in the art, techniques for heat removal are generally categorized either as convection or conduction techniques. Convection generally refers to the transfer of heat from one location to another by the movement of a transport medium, such as a fluid or air. In conduction techniques, heat generally diffuses through a material substance.
Conventional techniques for heat removal in transducers are natural convection and forced convection. Generally, natural convection in a transducer refers to the transfer of heat by the natural movement of air and forced convection refers to the transfer of heat by the forced movement of air created by a blower or fan. The technique of forced convection may provide adequate cooling; however, the reliability of a remotely located fan is cause for concern. The technique of natural convection is simple and reliable; however, it is generally only suitable for relatively low power applications.
Another convection cooling technique which is suitable for high power operation is evaporative cooling using a fluid with low boiling point and condensing point temperatures. This technique includes the use of a container disposed over the ceramic stack assembly in which wicks connect to all of the ceramic elements in order to transport heat from the elements to the fluid having suitable thermal properties. When the temperature within the ceramic stack assembly rises, the fluid transported on the wicks evaporates, providing the necessary cooling. However, in the case of evaporative cooling, the complexity of the apparatus may decrease reliability. The wicks which carry the fluid have limited fluid carrying capacity with respect to the ceramic stack surface area they contact. This limited capacity may result in non-uniform or decreased effectiveness of the technique, particularly at high operating power levels. Additionally, the capillary action of some wicks may be degraded when the transducer is operated at various physical orientations.
Thus, it would also be desirable to have a structure for cooling a flextensional transducer which is sufficiently simple in order to maintain reliability, manufacturability, and servicability of the transducer. The heat dissipation structure should also be effective at high operating power levels and maintain its effectiveness regardless of transducer orientation.